Reactive metabolites of bromobenzene are capable of leaving the intact hepatocyte and becoming covalently bound to externally added protein. Varying the amounts of externally added protein increases the amount of covalently trapped material, enabling an estimation of the fraction of the compound which can escape the liver and reach extrahepatic binding sites. As much as 35% of the reactive bromobenzene-3, 4-oxide is capable of escaping the intact hepatocyte.